In the games

Pokémon Diamond and Pearl

The Eterna Gym consists of a green foyer, past which is a greenhouse room where an indoor forest is arranged in a maze-like pattern. The Trainers of the Gym hide and wait behind trees and natural surroundings, and Gardenia will not accept challenges until all of her students are defeated. Every time a Trainer is defeated, they give a hint about where another foe is. Trainers of the Gym are fond of using status-inducing moves such as Poison Sting, Leech Seed, and Stun Spore.

Pokémon Platinum

In Pokémon Platinum, the entire Gym has been redesigned. The Gym now contains four areas with a giant clock in the middle, separating these areas. For every Trainer the player defeats, the hands of the clock move in a certain direction. Once the clock's hands spin to the north, south, east or west direction, the player can walk onto them and progress to other areas of the Gym. A set of sprinklers also turns off, allowing the player to exit the Gym and return later. Gardenia awaits the player in the northern area of the Gym (when the larger hand reaches 12 o'clock).

In the anime, the Eterna Gym is modeled like a large indoor garden, complete with trees and grass. Many different kinds of Grass-type Pokémon live there and are taken care of by Gardenia and her helpers. Unlike other Gyms, the Eterna Gym does not have an official battlefield. Instead, Gym battles take place in a large clearing in the middle of the Gym.

Pokémon used in Gym

Cherubi

Gardenia's Cherubi was first seen in The Grass-Type Is Always Greener! when Gardenia used it to battle Ash's Turtwig in an unofficial battle. It displayed its high speed, however it emerged that Gardenia was just trying to get a read on Turtwig's abilities before she recalled it.

It later appeared during Ash and Gardenia's Gym battle during The Grass Menagerie!, where it was able to outspeed Turtwig's attacks using its high speed and forced Ash to recall it. It then went up against Staravia where Ash was able to blind it using the sunlight and then Staravia was able to knock it out.

Roserade was the final Pokémon used by Gardenia against Ash. It was able to easily knock out Turtwig using a Weather Ball powered by the bright sun. When it went up against Ash's Aipom, it was unable to keep up with the active Pokémon. It was eventually knocked out with a combination of Focus Punch and Swift. It also reappeared in flashbacks in DP048 and League Unleashed!.

Later in Once There Were Greenfields, after battling alongside James and Cacnea in a Tag Battle, Gardenia knew that Cacnea had the ability to learn Drain Punch, so offered to take on Cacnea to help it reach its full potential. After much soul searching James agreed and handed Cacnea over. Gardenia soon helped Cacnea master Drain Punch and she used it in a Gym battle against Barry where it proved to be a strong opponent.

In the manga

Eterna Gym in Pokémon Adventures

In the Pokémon Adventures manga

The Eterna Gym appeared in Ring Around the Roserade II, where Platinum challenged Gardenia. Due to the great distance between the two sides, Platinum's Pokémon were unable to reach Gardenia's Cherubi and Roserade, while Roserade was able to use its vines to strike Platinum's Pokémon from underground. Eventually, however, Platinum figured out a way to use Gardenia's strategy to her own advantage, and had her Prinplup use the holes left behind by Roserade's vines to release an underground Blizzard attack at Gardenia's Pokémon, defeating them and earning Platinum the victory.